Optical phase measurement system and method

ABSTRACT

A method for use in optical measurements on patterned structures, the method including performing a number of optical measurements on a structure with a measurement spot configured to provide detection of light reflected from an illuminating spot at least partially covering at least two different regions of the structure, the measurements including detecting light reflected from the at least part of the at least two different regions within the measurement spot, the detected light including interference of at least two complex electric fields reflected from the at least part of the at least two different regions, and being therefore indicative of a phase response of the structure, carrying information about properties of the structure.

TECHNOLOGICAL FIELD AND BACKGROUND

As semiconductor technology progresses, shrinking device dimensions hasbecome an increasingly complex task. Complementing metrology tools,allowing similar improvements in measurement capabilities, are criticalfor the continual process of this development.

Optical metrology can acquire highly accurate and precise information onthe geometry and material properties characterizing patterned structureswith small dimensions of the pattern features (critical dimensions).Several physical quantities are commonly measured by optical metrologyand in particular by optical critical dimensions (OCD) techniques. Forexample, optical reflectometry measures the reflection intensity for abroad spectrum, over a single (or small set) of incidence directions anddifferent polarizations. Ellipsometry allows, in addition, access toinformation on the relative phase between different polarization states.

Another important attribute of light scattered from a patternedstructure is its phase, namely a relative phase between incident andreflected light beams. This phase can be measured using a variety ofinterferometry techniques. These methods are based on separating thelight beam into two parts so that only one part interacts with (isreflected from) the sample. The reflected light is then re-interferedwith the second part of the beam (“reference beam”), which did notinteract with the sample, and a difference in the optical path lengthtraversed by these two parts is accurately controlled. The interferencepattern formed by interference of these two light components in adetection plane is then used to extract the spectral phase.

Existing approaches for measuring phase, including interferometry, arehighly delicate, require special measurement apparatus with a reference,and are highly sensitive to environment (e.g. system vibrations).Consequently, such methods are not regularly used for in-line OCDmetrology, whereas the more robust methods of reflectometry andellipsometry are customary.

GENERAL DESCRIPTION

There is a need in the art for a novel approach in optical measurementson patterned structures with small features of the pattern. As indicatedabove, an important attribute of light reflected/scattered from apatterned structure is its phase, i.e. a relative phase or phase shiftbetween incident and reflected light beams. Such a phase shift isdifferent for interactions of incident light with different regions ofthe structure (e.g. differently patterned regions, patterned andunpatterned regions, regions with different material layers, etc.).

Further, a phase shift is different for different wavelengths ofincident light, and can thus present a spectral phase shift. FIG. 1schematically illustrates the principles of “spectral phase” effect. Asshown, two light beams of different wavelengths λ₁ and λ₂ are incidenton the same illuminating spot including two different regions of apatterned structure (patterned structure generally could includedifferent patterned and unpatterned regions) and are reflected fromthese different regions with different phase shifts Δ

(λ₁) and Δ

(λ₂) respectively. It should, however, be understood that when using thesingle illumination wavelength,λ, for illuminating two differentregions, phase shifts Δ

₁(λ) and Δ

₂(λ) will be different because of different effects induced by differentregions. These phase shifts carry significant information on theproperties of the structure. Thus, generally speaking, illumination oftwo (or more) different regions of a structure causes two (or more)different interactions between incident light and the structure,resulting in different phase shifts being indicative of the propertiesof the structure.

The present invention provides a novel measurement method and system foroptical measurements on patterned structures. The technique of theinvention provides a novel approach for measurement of the relativephase between an incident beam and a reflection thereof (i.e. phaseresponse, e.g. spectral phase), and several specific implementations ofthis capability. This novel approach is as so-called“self-interferometry” approach, based on detection of interferencepattern formed by

interference of light responses of different regions of the structure tocertain illumination (which may be single or multiple wavelengths), andanalyzing the detected light to determine the phase shift, to therebyenable extract data about the structure.

This technique can be implemented for various wavelengths and cangenerate a full spectrum phase differences per wavelength, which istermed here “spectral phase”, which can be used as highly valuableinformation for optical characterization (and importantly for OCDmetrology). Specifically, this technique is inherently robust againstsystem vibrations, avoiding the dominant causes for measurementinaccuracies typical of other interferometric systems.

It should be noted that the term “patterned structure” used hereinrefers actually to a layered structure, where the surface of thestructure has different regions. The term “different regions” refers topatterned and unpatterned (homogeneous) regions, and/or regions withdifferent patterns, and/or regions with and without certainlayer(s)/film(s) respectively.

It should also be noted that in the description below, the term“measurement spot” refers to detected light, reflected from anilluminating spot on a structure, and carrying measured data. Themeasurement spot is actually a result of interaction of incident lightwith the structure within an illuminating spot and a spot (detecting orimaging spot) formed by detected reflection from the illuminated spot ona detector. Accordingly, an optical measurement is performed with ameasurement spot which is configured such that a detected spot is formedby light reflections from an illuminating spot, at least partiallycovering at least different-type regions of a structure.

Thus, according to one broad aspect of the invention, there is provideda method for use in optical measurements on patterned structures. Themethod comprises performing a number of optical measurements on astructure with a measurement spot configured to provide detection oflight reflected from an illuminating spot at least partially covering atleast two different regions of the structure; said measurementsincluding detecting light reflected from said at least part of the atleast two different regions within the measurement spot, the detectedlight comprising interference of at least two complex electric fieldsreflected from said at least part of the at least two different regions,and being therefore indicative of a phase response of the structure,carrying information about properties of the structure.

The illumination includes either normal or oblique incidence mode, orboth of them.

Preferably, in order to improve extraction of phase data, more than onemeasurement is taken with the same measurement spot configuration (e.g.the same-size illuminating spot) but different coverage of the at leasttwo different-type regions of the structure. This enables differentialphase measurements.

According to another aspect of the invention, there is provided a systemfor use in measurements on patterned structures, the system comprisingan optical measurement device comprising:

an illumination unit configured and operable for focusing light onto anilluminating spot on a structure at least partially covering at leasttwo different regions of the structure to thereby cause lightreflections from said at least two different regions; and

a light detection unit collecting light including the light reflectionsof said at least part of the at least two different regions, thecollected light therefore comprising an interference pattern formed byinterference of at least two complex electric fields reflected from saidat least two different regions, and being therefore indicative of aphase response of the structure, carrying information about propertiesof the structure.

As indicated above, preferably, at least two optical measurements aretaken, with different at least partial coverage of said at least twodifferent regions of the structure. These at least two opticalmeasurements with the different at least partial coverage of the atleast two different regions are performed either sequentially, or inparallel (using a position sensitive sensor in the detection unit).

In some embodiments, the measurements are performed with at least twodifferent wavelengths of illumination. In some embodiments, themeasurements are performed with multiple wavelengths of illumination,and the detected light is therefore indicative of a spectral phaseresponse of the structure. The detection light may be in the form of aposition and wavelength dependent signal.

In some embodiments, the illuminating spot is of a significantlydifferent size than a spot formed by the detected light.

The optical measurement device is typically configured for datacommunication with a control unit. The control unit is configured andoperable for processing data indicative of detected light response anddetermining relative phase of light reflected from the at least twodifferent regions, thereby enabling determination of properties of thestructure.

The control unit may utilize data indicative of mixing coefficientsaffecting the detected phase response, for the determination of therelative phase. The mixing coefficient may be provided as known data, ormay be calculated during the measurements.

For example, at least two optical measurements are performed while withshifted positions of the measurement spot, thereby controllablymodifying values of mixing coefficients affecting the detected lightresponse, and increasing amount of information about the structure inthe detected phase responses. Alternatively or additionally, at leasttwo optical measurements are performed with different values of at leastone measurement condition, thereby controllably modifying the detectedphase response, and increasing amount of information about the structurein the detected phase responses.

In some embodiments, the optical measurement device is configured andoperable for separating between TE and TM polarization components in thedetected light response.

According to another broad aspect of the invention, there is provided aninterferometric method comprising: performing one or more opticalmeasurements comprising: illuminating a structure with an illuminatinglight spot configured for at least partially covering at least twodifferent-type regions of the structure; and detecting light reflectedfrom the structure and including at least two different light responsesof said at least part of the at least two different-type regions, thedetected light comprising an interference pattern formed byself-interference of at least two complex electric fields reflected fromsaid at least two different regions, and being therefore indicative of aphase response of the structure, carrying information about propertiesof the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosedherein and to exemplify how it may be carried out in practice,embodiments will now be described, by way of non-limiting example only,with reference to the accompanying drawings, in which:

FIG. 1 schematically illustrates interaction of light beams withdifferent wavelengths with a sample, showing different phase shifts ofreflections of these beams from the sample;

FIG. 2 exemplifies the principles of self-interferometry approach of theinvention, where a measurement spot integrates information from a fewdifferent-type regions of the sample;

FIG. 3A is a block diagram of a measurement system of the invention;

FIG. 3B shows schematically an optical scheme for simultaneous spectralmeasurements from different spot locations, thus allowing for spectralphase extraction from a single-shot measurement;

FIG. 4 exemplifies the spectral reflectivity and phase dependence on afilm thickness for a structure formed by a thin SiO₂ film on top of Sisubstrate; and

FIG. 5 exemplifies a metrology technique using spectral phasemeasurements, where the spectral phase difference between two differentregions is determined by geometrical and/or material difference betweenthem, and can thus be used as a highly sensitive probe.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 described above schematically illustrates an example ofinteraction of light beams of two different wavelengths with a patternedstructure, showing different phase shifts of reflections of these beamsfrom the structure.

The present invention provides a measurement system suitable formeasurements (including metrology) on patterned structures, whichutilizes interference of light beams returned from different illuminatedregions of a structure, enabling robust and accurate phase measurements,e.g. spectral phase measurements. This approach is applicable tostructures including large homogeneous (or approximately homogeneous)regions, which is the common case for OCD metrology. The requirement forhomogeneity here means that the measured structure in each region iseither a periodic structure of largely identical elements (features of apattern) with a pitch smaller than the optical resolution of the system,or a truly spatially uniform layered stack. Optical resolution of anoptical system is known as a diffraction limited spot and is determinedas λ/NA, where λ is the wavelength of an illuminating beam and NA is thenumerical aperture of the optical system. The specific size andhomogeneity conditions are discussed in more detail below.

Reference is made to FIG. 2 schematically illustrating the principles ofa measurement technique of the present invention. As shown in thefigure, a patterned structure 10 has regions of different types, i.e.differently patterned, or patterned and non-patterned regions, ordifferent layered regions. Two such different regions R1 and R2 areshown in this specific not limiting example, where the region of type 1,R1, is a patterned region including a periodic array of repeatingstructures, and region of type 2, R2, is a uniform periphery surroundingregion R1, i.e. R2 is a non-patterned region. An illumination scheme(measurement site and measurement spot configuration) is selected suchthat an illuminating spot MS covers (at least partially) the two regionsR1 and R2 of different types.

It should be understood that the measurements scheme of the presentinvention is not limited to this specific example, and an illuminatingspot may cover more than two regions and thus a measured signal can becollected simultaneously from more than two regions. This is a“self-interferometry” measurement scheme, where the measurement spotintegrates information from a few regions of different types. Forsimplicity, in the following description, the situation where only tworegions of different type reside inside the measurement spot will bediscussed.

Measured signal, formed by light components returned (reflected) fromdifferent regions, depends on the electric-field response (returnedelectric-field) from each region, as well as on the relative phasebetween them. It should be noted that in the description below suchlight responses are referred as being associated with reflections fromthe illuminated regions. However, it should be understood that theinvention is not limited to specular reflection, and generally notlimited to reflection mode at all, and therefore the description in thisrespect should be interpreted broadly as “light returned/coming from theilluminated region in response to the illumination)”.

Thus, in this example, the measured signal depends on the complexelectric field E₁ reflected from region R1 and the complex electricfield E₂ reflected from region R2. Common reflectometers andellipsometers measure the reflected intensities, given by I=|E|², andconsequently cannot access the reflected spectral phase. Indeed, anellipsometer measures the phase difference between the two polarizationcomponents TE and TM, but cannot measure the phase effect or spectralphase effect illustrated in FIG. 1.

When a measurement scheme is configured such that an illuminating spotcovers (at least partially) more than one region, the reflectivity orintensity I_(Tot) cannot be simply expressed as a linear combination ofthe separate reflectivities (intensities), I₁ and I₂, of the tworegions, i.e.

I_(Tot)≠C₁I₁+C₂I₂   (1)

This is because the reflected fields from these regions interfere.

In practice, in many cases the measured intensity would be given by

I _(Tot)(λ)=C ₁(λ)|E ₁(λ)|² +C ₂(λ)|E ₂(λ)|²+

{C ₁₂(λ)|E ₁(λ)∥E ₂(λ)|exp((iΦ(λ))}.   (2)

In the above equations, C₁,C₂ and C₁₂ are prefactors which determine howthe reflected fields are mixed in the measured signal, as will bediscussed more specifically further below, and Φ is the phase difference(e.g. spectral phase difference in case plurality wavelengths are used)between the fields reflected from the two different regions.

In this expression, the measured reflectivity is a function ofwavelength, and is determined by the reflectivities (light responses) ofthe two regions (I_(1,2)), but importantly also on the interferenceterm, which depends on the spectral phase difference

(λ), generally on the phase difference due to the different interactionsat the different regions.

The measured intensity I_(Tot) holds information on the interferencebetween reflected fields from the different regions. The inventionprovides a so-called “self-interference” technique, utilizing thisdependence of the measured intensity on the interference betweenreflected fields from the different regions, to extract the phaseinformation, e.g. spectral phase. Thus, the self-interferencemeasurement technique of the present invention utilizes interferencebetween light responses (effects) of different parts of a single sample.It should be noted that the optical system of this self-interferencetechnique does not require an internal reference beam within the systemin order to extract the phase. In this respect, this method isinherently different from other interferometry approaches.

As mentioned above, the prefactors C₁,C₂ and C₁₂ determine how thereflected fields are mixed in the measured signal, and as such aretermed here is the ‘mixing factors’.

It should be noted that the mixing factors can be explicitly calculated,and depend only on the optical system characteristics, such as numericalaperture, main aberrations and illumination scheme (Koehler\critical)and the sample layout including lateral dimensions of the differentregions and their layout with respect to the measurement/illuminatingspot location. The mixing factors are independent of the details of thespecific structure or application within each region (i.e. independentof details of pattern and/or material composition(s) of layers(s)).

These factors can be a-priori calculated or calibrated independently ofthe application details. In the following analysis, the mixing factorsare assumed to be known.

It should thus be noted that the mixing factor, C_(i), is a certainfunction F₁(λ,OS_(charact), MS_(location)) of the wavelength λ, themeasurement optics OS_(charact), and the spot location MS_(location),and is independent of structure parameters. In other words, the mixingfactors are defined by the configuration of the optical measurementsystem and measurements conditions, and can thus be predetermined priorto taking actual measurements on the structure with said system and saidconditions. The reflected fields E₁ and E₂ are function F₂(λ,S_(par)) ofthe wavelength λ, and the structure parameters S_(par), but areindependent of the spot location: the entire dependence on spot locationis encapsulated in how the spot would sum the reflected fields, which isdetermined by the mixing factors. In addition, the reflected fieldsdepend on the illumination polarization (if illumination is polarized)and on the selected collected polarization (if determined by, e.g., ananalyzer). The mixing factors are entirely geometric, and independent ofpolarization.

The inventors have shown that such decomposition of the measured signalinto application-or structure-dependent parts (E₁,E₂) and optical systemand sample layout parts (C₁,C₂ and C₁₂) is valid in cases where thedistinct regions are significantly larger than the used wavelength,which is the case for OCD metrology.

The principles of the invention for phase metrology can be implementedwith various optical metrology techniques (e.g. spectral reflectometry,spectral ellipsometry). However, the optical system in this case shouldbe appropriately configured to satisfy required conditions for theillumination scheme/channel and in some cases also for the detectionchannel.

More specifically, for (spectral) phase based measurements, theillumination and detection channels can be configured to provide adesired configuration of a spot size on a structure under measurements.If the mixing factors are to be found through calculation, it might bebeneficial, although not necessary, that either the illumination orcollection geometrical spot is very large, and the other small. Asstandard in the design of optical systems, the geometrical spot sizesare determined by the illumination\collection field stops. One of thesespots is commonly required to be small, so that the measured region islimited and confined to the target of interest. However, when the otherspot is significantly larger than the coherence length (given by λ/NA),the mixing coefficients C₁,C₂ and C₁₂ become much simpler to calculate,and also less sensitive to specific optical attributes and imperfectionsof the optical system. Similarly, if the mixing factors are to be foundthrough calculation, it might be beneficial, although not necessary, touse Koehler illumination scheme, where a light source is placed at theback focal plane of the illumination channel. Again, this is not astringent requirement, but would simplify the calculation of the mixingfactors and their dependence on some optical attributes of the system.In some embodiments, it might be beneficial to separate and measurereflectivity (light response) of a structure to light of each of thepolarization states TE and TM, as these would provide additionalinformation about the structure. For structures which providesignificant polarization rotation, measuring the full Jones matrixentries can be additionally beneficial.

With regard to a measurement site, it should be noted that in someembodiments, good knowledge on the measurement location with respect tothe structure layout might be preferable, because positioning errorswould lead to errors in the assumed mixing factors, and consequentlyerrors in the interpretation of measured data. Consequently, it might bedesirable that an imaging channel provides accurate feedback on themeasurement spot location.

Further, if the mixing factors are to be found through calculation, thefact that the calculation of the mixing factors is influenced by systemaberrations should preferably be taken in account. In this case,low-aberration (high quality) optics can be used, or alternatively theseaberrations may be characterized (estimated) and corresponding data beused in the calculation of the mixing factors. The present inventionadvantageously provides that if the optical system aberrations are wellcharacterized, one can completely account for all system aberrationswhen calculating the mixing factors (separately from the calculation ofthe reflected field from the application).

It should be noted that, preferably, the optical measurements includetwo or more measurements with the same measurement spot (same-sizeilluminating and detecting spots) applied to the same two or moredifferent regions but with different at least partially coverage ofthese regions by the illuminating spot. This enables differential phasedetermination. Such measurements may be applied sequentially, byshifting the measurement spot, or in parallel using 2D sensor in thedetection unit.

Reference is made to FIG. 3A showing schematically, by way of a blockdiagram, a measurement system 100 of the invention. The system 100includes an optical measurement device 102 connectable (via wires orwireless signal transmission) to a control unit 104. The control unit104 is typically a computer system including inter alia datainput/output utilities 104A, memory utility 104B, and processor 104C. Insome embodiments, the control unit may also include various controllersfor controlling the operation of one or more optical elements of themeasurement device, such as a scan controller 104D, and/or illuminationcontroller 104E. The optical measurement device 102 includes a lightsource unit 106, a detection unit 108, and focusing and collectionoptics 110.

The optical measurement device 102 may be configured for operation withnormal incidence, in which case the illumination and detection channelsare partially overlapping, and accordingly the focusing and collectionoptics includes optical element(s) located in the common optical pathfor propagation of illumination light towards a structure andpropagation of light returned from a structure 10. Also, in someembodiments, the optical device 102 includes a splitting optics 112. Forexample in case of the normal incidence configuration, the splittingoptics may include a beam splitter/combiner for spatially separatingbetween the illumination and returned light beams. It should be notedthat splitting optics 112 may be used (irrespective of normal or obliqueconfiguration of the optical scheme) to perform spectral splitting oflight returned from the structure. Further, as described above, theoptical device 102 might include polarizer(s) 114.

The control unit 104 receives input measured data indicative of thedetected light returned from the measurement spot. The measured data maythen be analyzed to detect mixing factors, or as shown in the figure indashed lines, data indicative of the mixing factors may be supplied aspart of the input data (known value of mixing factors). The mixingfactors (a priori known or calculated from the preliminary measurements)are then used to interpret measured data and determine the spectralphase.

According to some embodiments of the invention, spectral phase isdetermined using several measurements taken sequentially at differentilluminating spot locations. This, however, imposes limitations on themeasurement speed, and therefore might lead to some throughputreduction.

An alternative approach, which is preferable for some applications, suchas automatic optical inspection/measurements, is to measure severallocations simultaneously. This approach would involve using a 2D sensorwhere different locations on the sample are simultaneously registered.

An example for an optical measurement device 102 configured with anoptical scheme which implements such a single-shot spectral phasemeasurements is shown in FIG. 3B. In this specific but not limitingexample, the optical scheme utilizes oblique incidence. Accordingly, thefocusing/collection optics 110 includes separate focusing and collectinglens units 110A and 110B (each including one or more lenses). LightL_(in) from the light source unit 106 is focused onto the structure 10by objective 110A, creating an illuminated/measurement spot MS. LightL_(re) returned (reflected) from the measurement spot MS is collected bycollection objective 110B. Also, in the present example, splittingoptics 112 is configured as a spectral splitter, and includes a grating(or prism) to spectrally break the light L_(re) so that differentwavelengths are focused on different positions on a 2D sensor 109 of thedetection unit 108.

The grating 112 is designed (has a predetermined pattern) such that eachwavelength reflected through the grating 112 creates a highly elongatedimage spot IS on the sensor 109 (in the detection plane), i.e. on oneaxis (the ‘λ’axis) the image spot is highly focused. As shown in thefigure, in this example, the grating 112 splits the collected returnedlight L_(re) into five light components of different wavelengths λ₁-λ₅resulting in respective five image spots IS(λ1)-IS(λ5) on the detectionsurface 109. Hence, along the respective axis, λ-axis in the presentexample, every pixel line of the 2D sensor 109 “reads” a narrow set ofwavelengths but the full spatial span of the measured spot MS in onedirection. In the other direction, the ‘x’ axis in the present example,the grating acts as a lens, imaging the measurement spot MS so thatdifferent pixels correspond to spatially distinct regions on thestructure. In such a scheme, the detection unit 109 and the splittingoptics 112 operate together as a position and wavelength sensitivedetector, and a large set of distinct locations on the sample can becaptured by a single measurement, allowing efficient spectral phaseextraction. It should be noted that such 2D sensors are known foroptical spectrometers.

It should be understood that the above-described measurement scheme is aschematic one and can be modified considerably, as long as eventually aposition and wavelength dependent signal is obtained. The design caninclude any set of additional polarizers, retarders or other opticalelements. It is also possible to use a normal-incidence design,utilizing a beam-splitter in the illumination and light collection path.

In addition, considering spectral filtering configuration, a separatelens can be placed before a simple (flat) grating, providing similarposition and wavelength sensitive functionality.

Another possibility is not to image the sample onto the 2D detector, butrather to image the back focal plane, so that different reflecteddirections are mapped to different locations on the 2D sensor. Turningback to FIG. 3B, in this configuration, the x axis would correspond todifferent reflection directions, and the system allows analysis of theangular dependence of the reflectivity. Such measurement scheme can beimplemented, e.g., by using a simple grating which spectrally breaksdifferent wavelengths in one axis (‘λ’), but does not alter the lightpath in the other axis (‘x’).

The above-described measurement technique of the invention can beeffectively used for OCD. Let us consider such a measurement asdescribed above, providing a mixture of two (or few) unknown fields (Eq.2). For every wavelength λ, there are three unknowns, |E₁|, |E₂|, andtheir relative phase

, and one measured quantity I_(Tot)(λ). It is possible to increase theamount of available information for example by shifting the measurementspot so as to modify the mixing coefficients C_(i). Alternatively, someoptical parameter(s) of the measurement device can be changed by a knowndegree (e.g. the illumination\collection numerical aperture), which willalso result in a modified measured spectrum. By taking several suchmeasurements, a set of measured spectra is obtained (here i stands forthe different spot location\different optical system I^(i) _(Tot)(λ)attributes), each satisfying the following condition:

I _(Tot) i(λ)=C ₁ ^(i)(λ)|E ₁(λ)|² +C ₂ ^(i)(λ)|E ₂(λ)|²+

{C ₁₂ ^(i)(λ)|E ₁(λ)∥E ₂(λ)|exp(iΦ(λ))}.   (3)

For each such measurement, the mixing factors can be calculatedindependently of the application (structure). Consequently, using N suchmeasurements, the values of |E₁(λ)|, |E₂(λ)| and

(λ) can be directly found, as long as N≥3 (so that there are at leastthree equations for these three unknowns). More measurements can be usedfor noise reduction and accuracy improvements.

Alternatively, if one (or several) of the measured regions is wellcharacterized, so that e.g. |E₁(λ)| is known, only two measurements canbe used to explicitly extract and

(λ), possibly decreasing the required acquisition time for thismeasurement.

As stated, this approach can be expanded to more region types,correspondingly requiring more measurements to allow solution for thedifferent reflectivities and relative (spectral) phases. For example, inthe case of three different locations, the measured signal would beexpressed as

I_(Tot) ^(i)(λ)=C ₁ ^(i)(λ)|E ₂(λ)|² +C ₂ ^(i)(λ)|E ₂(λ)|² +C ₃^(i)(λ)|E ₃(λ)|²+

{C ₁₂ ^(i)(λ)|E ₁(λ)∥E ₂(λ)|exp(iΦ ₁₂(λ))}

{C ₂₃ ^(i)(λ)|E ₂(λ)∥E ₃(λ)|exp(iΦ ₂₃(λ))}+

{C ₁₃ ^(i)(λ)|E ₂(λ)∥E ₃(λ)|exp(iΦ ₁₃(λ))}.   (4)

In this case, there are 6 mixing factors, associated with thecontributions of the reflected fields from each of the regions, and theinterferences between these fields. Generally, when N region types aresimultaneously measured, there would be N(N+1)/2 mixing terms.

The reflected spectral phase from a test site holds valuable informationon its scattering properties, and can hence be used as basis for opticalcritical dimensions (OCD) metrology. The relative spectral phasesbetween the different regions can be calculated for any sample usingstandard techniques for solving electromagnetic reflection problems(e.g. FDTD, RCWA, FEM, eigenmode expansion etc.). Let us consider asample/structure with known geometry, characterized by a set of unknownparameters. These parameters can be geometrical dimensions, thicknessesand material optical properties (i.e. refractive indices). For any setof values assumed for these parameters, the expected spectral phase (aswell as the spectral reflectivity) can be calculated. It should be notedthat the spectral phase includes additional information on the measuredsample, which is not fully characterized by the spectral reflectivityand/or by the ellipsometric phase.

Given a measured relative spectral phase, it can be compared to a set ofpre-calculated such spectral phase. By finding the combination ofparameters which produce the best fit between measured and calculateddata, the actual parameters of the measured structure can be determined.This approach is customary for OCD metrology, and is commonly applied tothe techniques mentioned above. Since in the process of measuring thespectral phase one also obtains the spectral reflectivities |E_(1,2)|²,it is natural and beneficial (although not obligatory) to use both thespectral reflectivity and the spectral phase in such fitting process.

It should be noted that identification of the ‘best fit’ solution wouldinvolve searching for the set of parameters, for which the calculatedsignature (e.g. spectra , phase spectra) is most similar to themeasured. However, considering that the phase and reflectivityinformation may suffer from different noise attributes, it may bebeneficial to weigh these in some nontrivial way. For example, it may bedesirable to assign more importance to phase signal than toreflecitivity (or vice versa), to have this weight wavelength dependentor to apply any form of nontrivial mathematical manipulation to the dataas part of the analysis.

The technique of the present invention allows measurement of test padswith dimensions smaller than the illuminating spot. This capability ishighly beneficial for metrology applications, when the illuminating spotis often many tens of micrometers in size, and metrology for regionssignificantly smaller than this dimension is required. One example forsuch need is the capability to measure test sites inside the patternedregion (‘in-die’), where test pads are restricted in their dimensions.Alternatively, such problems arise when the illuminating spot isespecially large, for example when highly oblique illumination anglesare used.

As explained above, by taking several measurements which mix thecontributions from within the test pad and its periphery with differentratios, it is possible to tell apart the separate reflectivities ofthese two regions. Specifically, it is possible to use this approach toisolate the reflectivity of the pad region, irrespective of its size.

As exemplified above, the surroundings of the region of interest(patterned region R1) are largely homogeneous (non-patterned region R2),so that for the different measurement locations only the mixing factorswill change, and not the inherent reflectivity. If the pad surroundingsare inhomogeneous, errors will be induced in the extracted fields.However, even if the surroundings are not homogeneous, it may bepossible to use more measurements to average out the position dependenceand accurately extract the reflectivity associated with the test pad.

The phase metrology technique of the invention can be used for thin filmanalysis. Thin film metrology targets accurate measurement of filmthicknesses in the range between 0.1 nm and up to a few tens of nm.

In this case, the film reflectivity weakly depends on its thickness,making reflectometry measurements difficult. In contrast, the spectralphase is expected to be highly sensitive to the film thickness, showing(approximately) linear dependence. In this connection, reference is madeto FIG. 4, showing spectral reflectivity and phase dependence on filmthickness for a specific example of thin SiO₂ film on a Si substrate. Inthe figure, graphs G1 and G2 correspond to the spectral reflectivity andspectral phase, respectively, as a function of the film thickness. Forfilm thickness in the range 0-10 nm, the spectral reflectivity varies by˜1% while the spectral phase varies by ˜10% suggesting significantlyimproved sensitivity.

It should be noted that with the “self-interferometry” measurementscheme it might be difficult to properly characterize an extended thinfilm structure: the measurement spot covers one region holding the thinfilm, and another region identical to the first but without the thinfilm. In that case, the phase difference between the two regions willstrongly depend on the film thickness. Alternatively a dedicated testregions can be devised to have two regions with different filmthicknesses, as is also detailed below.

Typically, optical metrology is applied to test sites comprised of arepeating array of the structure/pattern of interest. This structure(transistor, memory cell etc.) is characterized by multiple geometricaland material properties (thicknesses, dimensions, refractive indices,etc.) which influence the reflected signal. Naturally, the reflectedsignal is sensitive to all these parameters, while in practice only asmall subset of these parameters is of importance to monitor. Moreover,it is often the case that the sensitivity to the parameters of interestis relatively weak compared to sensitivity to irrelevant parameters,imposing stringent requirements on the measurement and modelingaccuracy. This sensitivity to irrelevant parameters poses a significantchallenge to OCD techniques.

The ability to measure the relative phase between different regionsallows for a unique kind of metrology approach. In this connection,reference is made to FIG. 5 exemplifying a specifically designed testpad/structure 10, with two different regions R1 and R2, which are bothpatterned but with different patterns. In this specific not limitingexample, region R1 holds one pattern, while region R2 (termed a“reference region”) holds a similar pattern as region R1, but with somemodification introduced to the parameter(s) of interest. In thisexample, the parameter of interest is the thickness of some internallayer L_(Int), which exists in the pattern of region R1 while is removedfrom the pattern in the reference region R2. An illuminating spot MScovers regions R1 and R2 and thus a light response from the illuminatingspot integrates the reflected fields from both regions, allowing forspectral phase extraction using the technique of the invention describedabove. While the reflected spectrum is determined by the entirestructure, the spectral phase difference between the two regions R1 andR2 is highly sensitive to the difference between them (specifically, ifno such modification is introduced, the spectral phase difference wouldbe 0).

This approach can be applied to various metrology reflectometry methods,e.g. spectral reflectometry and spectral ellipsometry. In addition, itcan be applied for the characterization of (essentially) any parameterof interest of a patterned structure, e.g. CD, pitch, materialproperties, side-wall angle etc.

It is similarly possible to change more than one parameter of thestructure in the two regions, in which case the spectral phasedifference will be sensitive primarily to those parameters which weremodified. Furthermore, it is possible to design a test pad comprised ofmore than two such regions, possibly allowing improved differentiationbetween different parameters of interest.

As evident from Eq. 2 above, the ability to separate system-related fromapplication-dependent contributions to the measures signal, i.e. lightresponse, allows a unique approach for characterization of the opticalmeasurement system. By measuring a well characterized structure, withspecific spatial (or angular) reflectivity properties (e.g. a knife-edgesample), the obtained (measured) and expected (modeled) signals can becompared. Any difference between the measured and expected signals isinevitably related to the optical system characteristics, for exampleits aberrations.

Since the influence of the optical system characteristics andaberrations can be directly used to derive the expected mixingparameters, it is furthermore possible to look for a set of aberrationswhich affect the measured signal. This can be accomplished by a varietyof fitting methods. For example, it is possible to calculate the mixingfactors expected for a variety of system aberrations, and identify thecombination of aberrations which provides a best fit for themeasurement. Alternatively, regression techniques could be used.

Another possible implementation is to measure the same location on thesame sample through two different optical configurations (e.g. differentobjectives). The differences between the measured signals can be relatedto the differences in optical characteristics, as discussed above.

Similarly, several measurements can be taken at different polarizations,enabling polarization-dependent characterization of the elements. Suchanalysis could be useful, for example, for the characterization ofstress effects (which is known to cause birefringence, i.e.polarization-dependent transmission).

More generally, such analysis can be done for any parameter of theoptical system which could be varied. As exemplified above, measurementscan be done for multiple wavelength and obtain a wavelength-dependentcharacterization. Alternatively, or additionally, the location and\ororientation of some optical element can be varied, and the opticalsystem can be characterized for varying placements of the opticalelement(s). Such characterization can then be used to optimize thesystem to any desired attributes.

Thus, the present invention provides a simple and effective techniquefor characterizing/measuring patterned structures. To this end, theinvented technique utilizes simultaneous optical measurement on aportion of the structure including two different regions, anddetermining the spectral phase of the light response from the structure.Measured data indicative of the light response from two differentregions (e.g. measured intensity) holds information on the interferencebetween returned (reflected) fields from the different regions, and thisdependence is used to extract the spectral phase.

What is claimed is:
 1. A method for use in optical measurements on patterned structures, the method comprising: performing a number of optical measurements on a structure with a measurement spot configured to provide detection of light reflected from an illuminating spot at least partially covering at least two different regions of the structure, said measurements including detecting light reflected from said at least part of the at least two different regions within the measurement spot, the detected light comprising interference of at least two complex electric fields reflected from said at least part of the at least two different regions, and being therefore indicative of a phase response of the structure, carrying information about properties of the structure. 